The present invention relates to a stereo signal demodulation circuit and, particularly, to a demodulation circuit for the compatible quadrature AM stereo signal which is compatible with both stereo and monaural receivers.
The AM stereo signal of the compatible quadrature AM system is one of a stereo signal system, which is expressed as follows: EQU stereo signal-e.sub.s =[{1+k(L.sub.(t) +R.sub.(t))} cos .omega..sub.c t+k{L.sub.(t) -R.sub.(t) } cos (.omega..sub.c t+.pi./2)] cos .phi.(1) EQU .phi.=tan.sup.-1 k{L.sub.(t) -R.sub.(t) }/[1+k{L.sub.(t) +R.sub.(t) }](2)
where: L.sub.(t) and R.sub.(t) are the left channel signal and the right channel signal, respectively;
.omega..sub.c is an angular frequency of a carrier signal, and
k is a modulation factor.
The compatible quadrature AM stereo signal expressed by equations (1) and (2) contains a first signal component obtained by amplitude modulating the carrier signal cos.omega..sub.c t with a signal corresponding to the sum of the left and right channel signals. A second signal component is obtained by amplitude modulating the signal cos (.omega..sub.c t+.pi./2), which is out of phase by 90.degree. from the carrier signal, with a signal corresponding to the difference between the left and right channel signals. The level of the composite signal of the first and second signal components is modulated with cos.phi., as indicated in the equation (1), to make possible demodulation of the composite signal in a monaural receiver.
A demodulation circuit for the stereo signal represented by equations (1) and (2), is conventionally a product-demodulation circuit, known in the art, which utilizes a differential amplifier. The conventional product-demodulation circuit tends, however, to produce distortion in the final audio signal because the left and right channel signals L.sub.(t) and R.sub.(t) are obtained as half-wave rectified signals.